3.1 Pond Culture
3.2 Floating Raft Culture
3.3 Mud Flat Culture
3.4 Pen Culture
3.5 Inshore Cage Culture
3.6 Offshore Cage Culture
3.7 Artificial Reefs
3.8 Sea Ranching and Restocking
The choice of a particular cultural system for sea farming or sea ranching depends on several factors such as existing natural conditions, availability of financial support and education/training level of fishermen. In China, several cultural systems are now commonly adopted (Table 27).
Table 27. Sea farming and sea ranching systems in China
System |
Cultured organisms |
Production scale |
Pond culture |
Fish, shrimp, crab, molluscs, Gracilaria
spp. |
Commercial |
Floating raft culture |
Seaweed, scallop, oyster, abalone |
Commercial |
Mud flat culture |
Laver, razor clam, hard shell clam, oyster |
Commercial |
Pen culture |
Abalone, shrimp |
Commercial |
Inshore cage culture |
Fish |
Commercial |
Offshore cage culture |
Fish |
Commercial and trials |
Tunnel culture |
Abalone |
Small scale |
Submerged cage culture |
Abalone, fish |
Small scale (trials) |
Indoor tank culture with water recirculation |
Flounder, turbot and other fish, abalone |
Commercial and trials |
Sea bottom culture and sea ranching |
Abalone, Japanese scallop, giant cockle, sea cucumber, sea
urchin |
Commercial |
Stock enhancement |
Chinese shrimp, red sea bream, flounder, large yellow
croaker, mullet, jellyfish |
Trials |
A high proportion of the area under cultivation is devoted to pond culture in marine and brackish waters. Most ponds are built in mud flat areas and used for farming shrimp and crab. A smaller area produces fish, molluscs and seaweed (Table 28).
Ponds are fed sea water either as tidal ponds or by pumping. Figure 20a shows typical coastal ponds for shrimp and finfish farming. Figure 20b shows ponds for farming mud crab, enclosed by fences to prevent crabs from escaping.
Since the early 1980s, pond culture developed rapidly throughout the country not only because its technology was relatively simple and it required less investments but also because of the traditional knowledge of fish pond culture which existed in China. An ample supply of penaeid shrimp postlarvae and profitable economic returns from shrimp culture further stimulated development.
Table 28. Areas used for sea farming/ranching, 1994-1998 (in ha)
Zone |
1994 |
1995 |
1996 |
1997 |
1998 |
Total area |
653 530 |
715 750 |
822 073 |
937 933 |
1 004 407 |
Shallow seas |
113 390 |
131 760 |
114 184 |
167 852 |
195 211 |
Bays/lagoons |
169 530 |
159 420 |
174 770 |
180 431 |
190 626 |
Mud flats |
370 610 |
424 570 |
533 119 |
589 650 |
618 570 |
Shrimp culture |
150 710 |
136 610 |
143 970 |
178 778 |
194 624 |
% total area |
40.66 |
32.17 |
27.00 |
30.31 |
31.46 |
Figure 20. Coastal pond culture of shrimp, fish and crab
(a) Ponds for marine shrimp and fish farming
(b) Ponds for mud crab farming
There is a long history of using floating rafts for shallow-sea farming. This system can be used for a variety of species such as seaweed (kelp and laver), filtering organisms (scallop, oyster, mussel) and abalone, combined with culture in small submerged cages (lantern cages).
Figure 21. Floating rafts for shallow-sea farming
(a) Oyster farming in southern China
(b) Kelp, scallop or oyster farming in northern China
The system shown in Figure 21 is widely adopted for farming kelp, oyster, scallop, abalone, etc. These species being either autotrophic or filtering organisms, there is no pollution of the environment, particularly with kelp farming as mentioned before. The system shown in Figure 22 is typically used in the Zhejiang Province for farming oyster, the raft being made of bamboo, an excellent natural material.
Figure 22. Bamboo floating raft for oyster farming
(a) Details of the bamboo infrastructure
This system is especially suitable for farming benthic species such as blood cockle (Tegillarea granosa), hard-shelled clam (Meretrix meretrix), manila clam (Ruditapes philippinarum), razor clam (Sinonovacula constricta) and seaweed such as Porphyra spp. It is very popular in China because it does not require large quantities of food and it does not pollute the environment.
Before the 1980s, mud flat culture was limited to particular areas because of the difficulty to obtain seed which had to be collected from wild stocks. The seed supply is now greatly improved. For blood cockle, Cyclina sinensis, seed are produced on a commercial scale. For other species including hard-shell clam and manila clam, protecting wild broodstock has been an effective way to enhance seed production. For razor clam, which has been farmed in China for decades, wild seed are collected following a rather sophisticated system involving site selection, smoothening the spawning area, forecasting seed settling, collecting and transporting seed.
In general, the mud flat ecological system is rather vulnerable, lying at the interface between sea and land. For example, in some areas, mud flats have been blindly exploited, even overexploited, for shrimp farming which has resulted in destroying the original ecological system and shrimp farming itself. This has occurred too frequently which has led to give priority to the protection of mud flat and mangrove forest areas.
Sites for pen culture are selected in intertidal areas where low fences and water gates are built (Figure 23). At low tide, water depth drops to a minimum of 50 to 80 cm inside the pens. As the next high tidal water starts coming, it enters the pens where water depth increases progressively again.
This culture system is popular in some provinces of China, but problems are the same as with pond culture, pollution having serious impacts on the environment. The development of this system should therefore be strictly controlled and confined to selected areas.
Figure 23. Pen culture in the Zhejiang Province
Inshore cage culture is very popular world-wide, especially in Asia. Advantages include low investment and easy routine management. But this system is one of the main sources of inshore pollution and the main cause for red tides. It has been reported that, in areas where inshore cage culture is well developed, phosphorus content of the sea bottom is 2 000 times higher than the legal value accepted for environmental protection. Policy makers have begun to notice the problems and to issue regulations controlling the development of this cultural system, to protect the natural environment and the aquaculture industry itself.
Crowded inshore cages are shown in Figure 24. As most farmers use trash fish to feed cultured fish with a high commercial value, it does not take long before a great quantity of faeces and food residues accumulate on the sea floor and pollute the entire area.
Figure 24. Floating cage culture in inshore areas
(b) Crowded cages inside a bay
Another problem is that floating cages cannot resist strong winds and waves. After a typhoon, few cages are still functional and many of the farmed animals have escaped (Figure 25). In the near future, offshore cage culture (Section 3.6) should become the main cage culture system.
Figure 25. Damaged inshore cages after a typhoon
China experiences typhoons seasonally, especially in its southern regions from the Zhejiang Province to the Hainan Province. Even if typhoons are only occasional in the northern regions, losses can also be very serious.
Traditionally, only floating cages are used in inshore waters. Due to pollution and overexploitation of these waters, fish cage farming should be moved offshore. At present there are two types of offshore cages recently introduced into China. One type is a sinkable cage (Figure 26a), the other being a plastic gravity cage (Figure 26b).
Researchers and farmers have been encouraged to design new models of cages which would be able to withstand typhoons and strong sea currents. In the near future, offshore cage culture should develop rapidly.
Figure 26. Offshore cage culture
Another way of sea ranching is using artificial reefs (Figure 27). This method was experimented in the 1980s for five years. Theoretically, it contributes to the enrichment of fishery natural resources. But its success depends on many factors such as site selection, shape of the artificial reefs, desirable species and other environmental conditions. Financial support is also important because returns on investments should be considered on the long term only. The development of this system has been stopped.
Figure 27. Artificial reefs for sea ranching
(a) Assembly of an artificial reef
(b) Installation of an artificial reef at a selected site
(c) Artificial reefs awaiting to be installed offshore
In the 1970s, Dr C. K. Tseng presented a series of talks and published a thesis entitled Sea ranching and sea farming. He defined ranching as a culture system where ... at first, the crop is reared in restricted areas, then its juveniles are released into the natural environment and finally its adults are fished from this natural environment.
Since the 1970s, sea ranching was tried with several species in various sites as shown in Table 29.
Shrimp of the species Penaeus chinensis were restocked on a commercial scale, especially in the late 1980s and early 1990s, at the annual rate of more than a milliard juveniles. Due to unknown reasons, this activity was stopped. It was observed that recapture rate of tagged shrimp depended on the individual size of stocked shrimp as well as on the duration and location of the restocking programme (Tables 30 and 31). Tabulated figures show that recapture rate varied greatly. The highest rate reached 1.88 percent, the lowest only 0.001 percent. It seems that some unknown factors influence recapture also.
Table 29. Sea ranching trials in China
Species |
Site |
Duration |
Remarks |
Penaeus chinensis |
Shandong, Liaoning and Zhejiang |
1980s to 1990s |
Restocking juveniles 3 cm long on a commercial
scale |
Paralichthys olivaceus |
Beidaihe, Hebei |
1980s to 1990s |
Experimental |
Liza haematocheila |
Beidaihe, Hebei |
1980s to 1990s |
Experimental |
Pagrosomus major |
Beidaihe, Hebei |
1980s to 1990s |
Experimental |
Haliotis discus hannai |
Shandong, Liaoning |
Since 1980s |
Restocking young abalone 3 cm long for commercial
purpose |
Stichopus japonicus |
Hebei, Shandong |
Since 1980 |
Restocking young sea cucumber (1-2 cm long) for commercial
purpose |
Patinopecten yesoensis |
Liaoning |
Since 1990s |
Restocking young scallop (3 cm long) for commercial
purpose |
Rhopilema esculentum |
Liaoning |
1990s |
Experimental with juveniles |
Year |
Location |
Juveniles size |
Stocking number |
Recapture number |
Recapture rate |
1985 |
Weihe estuary |
40.1 |
26 913 |
30 |
0.11 |
1985 |
Yellow River estuary |
40.1 |
46 438 |
73 |
0.16 |
1985 |
North of Liaodong Bay |
46.9 |
50 777 |
765 |
1.51 |
1986 |
Weihe estuary |
30 to 50 |
103 413 |
1 |
0.001 |
1986 |
Bohai Bay |
30 to 50 |
46 679 |
408 |
0.87 |
1986 |
Liaodong Bay |
30 to 50 |
95 272 |
19 |
0.02 |
* Based on data from Fan Ningchen, Yellow Sea Fisheries Research Institute, for P. chinensis
Table 31. Relationship between stocking size and recapture rate of shrimp*
Year |
Location |
Juveniles size |
Stocking number |
Recapture number |
Recapture rate |
1984-1985 |
Liaodong Bay |
35 |
36 408 |
125 |
0.34 |
1984-1985 |
Liaodong Bay |
52 |
30 000 |
345 |
1.16 |
1984-1985 |
Liaodong Bay |
63 |
34 369 |
713 |
2.07 |
1984 |
Laizhou Bay |
32 |
18 336 |
68 |
0.37 |
1984 |
Laizhou Bay |
40 |
18 394 |
121 |
0.66 |
1984 |
Laizhou Bay |
48 |
18 443 |
347 |
1.88 |
1985 |
Laizhou Bay |
32 |
26 110 |
177 |
0.68 |
1985 |
Laizhou Bay |
40 |
26 721 |
281 |
1.05 |
1985 |
Laizhou Bay |
48 |
26 148 |
364 |
1.34 |
* Based on data from Fan Ningchen, Yellow Sea Fisheries Research Institute, for P. chinensis
Figure 28. Restocking abalone juveniles into a selected site
(a) Young abalone remain attached to their rearing substrate
(b) Stocking young abalone into the selected site
Recapture rate for other species, such as abalone (Figure 28) and sea cucumber, is much more predictable than for shrimp. According to data recorded from commercial captures, recapture rate for abalone may be as high as 50 to 70 percent if stocking size is greater than 30 mm. If stocking size is less than 20 mm, recapture rate is very low. For sea cucumber, this rate depends more on site selection and improvement, such as increasing sheltered areas.
In general, sea ranching is a good system to enhance or to re-establish a population which has declined, but numerous factors influence results. Practically, at least four groups of factors should be taken into account:
- Species and natural population compositionResults show that sea ranching is in its infancy. There is still a long way to go to reach desirable goals. Cooperation in this field should be strengthened and enlarged.- Food web relationships of stocked species with natural population
- Environmental improvement
- Factors influencing recapture rate, including stocking location, duration of programme, size at stocking and potential predators.